1. Field of the Invention
The present invention relates to an optical scanner using a reflective image forming element.
2. Description of the Related Art
An optical scanner is known as a device for deflecting a light beam by an optical deflecting means such as a polygon mirror, etc. and converging the light beam as a light spot onto a scanned face by an image forming optical system such as an f.theta. lens, etc. to perform an optical scanning operation. As is well-known, the optical scanner is used in a writing section of each of an optical printer, a digital copying machine, etc.
Recently, a reflective image forming element having a concave mirror face is proposed and used as the image forming optical system or a portion thereof for converging the deflected light beam provided by the optical deflecting means as a light spot onto the scanned face. A reflective image forming element having a function for moving the light spot at an equal speed is also proposed. This function corresponds to the f.theta. function of an f.theta. lens.
When such a reflective image forming element is used, the reflective image forming element can be manufactured easily and cheaply in molding processing with respect to plastic.
However, since the reflective image forming element is formed by plastic, the reflective image forming element is easily influenced by temperature so that the optical scanning operation is influenced by a change in size of the reflective image forming element caused by a change in temperature. This influence greatly appears mainly as an error in magnification and a change in linearity of an image.
Recently, the mirror face is formed in an aspherical shape, etc. to improve optical performance of the reflective image forming element by correcting field curvature and linearity of the image. In this case, a toroidal lens, etc. is also used to perform the optical scanning operation with preferable accuracy. However, no optical scanner taking measures with respect to the above influence on temperature is known.